The present invention relates to the field of video broadcasting, particularly in a cable television system. More specifically, the present invention relates to an improved, multiple channel upconverter for upconverting television signals to an appropriate transmission frequency prior to transmission, e.g., over a cable television system.
Cable television systems allow subscribers access to dozens or even hundreds of channels of television programming. This wide variety of programming accounts for the great popularity of cable television. Additionally, the current trend is for cable television systems to provide additional services such as premium channels, pay-per-view programming, video-on-demand programming and even internet access. Providing video-on-demand programming, in which the subscriber can request transmission of a program, e.g., a movie, at any time, places particular burdens on the operation of a cable television system.
In a conventional cable television system, a cable television signal is transmitted from a central facility known as a headend. The signal from the headend travels over the cable television network to the homes or offices of individual subscribers where it is received and used. Typically, each subscriber will have a set-top terminal, a box of electronic equipment, through which the cable television signal is processed after being received through a co-axial cable wall outlet. The set-top terminal is connected to the subscriber""s television set and provides the cable television signal to the television for use by the subscriber. In advanced cable systems, the set-top terminal can also signal or send messages xe2x80x9cupstreamxe2x80x9d to the headend. Consequently, such a terminal can be used by the subscriber to electronically order a video-on-demand program directly from the headend.
Cable television signals are typically generated at an intermediate (IF) frequency that is unsuitable for transmission over the cable television network. Consequently, a device called an upconverter is used at the headend to upconvert the intermediate frequency signal to a signal in the very high frequency (VHF) or ultra high frequency (UHF) range suitable for transmission over the cable television system.
Conventionally, a separate upconverter is required at the headend for each television channel being transmitted over the cable system. Additionally, upconverter units are commonly built and sold as composite units that contain two independent upconverters. Thus, one upconverter unit may include two independent upconverting circuits and handle two of the channels of the cable television system.
In a simple upconversion technique, an incoming baseband or IF signal is multiplied in a mixer with a signal produced by a local oscillator, e.g., a carrier signal. The resulting output of the mixer is an upconverted output signal that occupies a desired VHF or UHF channel or frequency band. Typically, a filter is used to pass one sideband of the mixing process that falls within the desired VHF or UHF channel or frequency band, and to block any undesired sidebands or an oscillator signal at the output of the mixer from being transmitted.
The problem with this single mixer upconverter is that generally at least one of the upper and lower sidebands and the oscillator signal fall within the higher VHF or UHF channel. As a result, a special bandpass filter is needed to just pass, for example, the upper sideband signal and reject the lower sideband and oscillator signal. Alternatively, a well-known tracking filter can be used that only tracks the desired output signal. Both of the special bandpass and tracking filters are difficult and expensive to implement.
These problems can be overcome by implementing a two-stage upconverting technique with appropriate intervening filtering. As shown in FIG. 1, a basic two-stage upconverter includes two local oscillators, two mixing stages and appropriate intervening filters.
The input signal (106) is an intermediate frequency signal, typically 41 to 47 MHz. After filtering with a filter (101), the input IF signal is input to the first-stage mixer (102). The mixer (102) also receives a signal (110) from an oscillator (108). The mixer (102) multiplies the incoming IF signal (106) with the signal (110) from the oscillator (108) to produce a high intermediate frequency signal. Generally, the signal (110) output by the oscillator (108) is fixed as to its frequency. The oscillator (108) may, however, be able to accept and make small adjustments in the output frequency of the signal (110).
The mixed signal is output to a filter (103) and then provided to a second-stage mixer (104). The second-stage mixer (104) mixes the signal output by the first-stage with a signal (111) output by a second-stage oscillator (109). In this way, the second-stage mixer (104) converts the signal down to the appropriate frequency, VHF or UHF, at which it will be transmitted. This frequency will be different for each channel.
Consequently, the second-stage oscillator (109) should be controllable to output a signal (111) over the wide range of VHF and UHF channel frequencies, e.g., 50 to 1000 MHz, as needed. As illustrated in FIG. 1, a wide range voltage controlled oscillator (109) may be used for this purpose. This allows the system operator to set the upconverter shown in FIG. 1 to upconvert a television signal for any desired channel in the cable system. As noted above, a different upconverter is used for each channel.
The output of the second-stage mixer (104) may be filtered again in the filter (105). The result is an output signal (107) in the VHF or UHF range that is ready for transmission over the cable system. The output signal (107) created in this way is cleaner than those available from a one-stage upconverter.
A more advanced two-stage upconverter is described in U.S. Pat. No. 5,390,346, issued Feb. 14, 1995 to Daniel J. Marz of General Instrument Corporation. This upconverter is illustrated in FIG. 2. As shown in FIG. 2, the first and second-stage oscillators illustrated in FIG. 1 are replaced by oscillator synthesizers (201, 202). These synthesizers (201, 202) use a phase looked loop to synthesize the output of an oscillator, i.e., oscillating signals (110, 111). A reference signal (204, 205) is provided respectively to each oscillator synthesizer (201, 202) to which the phase of the output signal is locked by the phase looked loop circuitry of the synthesizer (201, 202).
Both the synthesizers are wide range synthesizers that can be controlled to output oscillating signals (110, 111) at any desired frequency over the VHF and UHF ranges, i.e., 50 to 1000 MHz. A control means (203) is provided to control the frequency of the output signal of the synthesizers (201, 202).
An expensive aspect of prior art upconverters, such as those described above, resides in the requirements placed on the second-stage oscillator and oscillator synthesizer. Because the second-stage oscillator determines the frequency and, therefore, the television channel, on which the output signal (107) will be transmitted, the second-stage oscillator must be able to output a signal (111) at any one of many closely located frequencies over the extremely broad range of the VHF and UHF bands. This is required to allow the cable system operator to use the upconverter for any desired channel. However, placing these demands on the second-stage oscillator increases it costs and can degrade its performance.
These limitations on prior art upconverters are exacerbated in the video-on-demand field. With video-on-demand, essentially a separate channel is used to provide the programming requested by the subscriber at the time the subscriber requests it. Thus, a separate upconverter is used to supply each program requested by a subscriber through a video-on-demand service. Consequently, as the number of upconverters increases to meet subscriber demand, the cost of each upconverter and the space if requires at the headend facility limit the ability of the cable system operator to expand the video-on-demand service.
Therefore, there is a need in the art for an upconverter and method of implementing the same that can more economically provide for upconversion of television signals in a cable network, particularly for video-on-demand service.
It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide an upconverter and method of implementing the same that can more economically provide for upconversion of television signals in a cable network, particularly for video-on-demand service.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.
To achieve these stated and other objects, the present invention may be embodied and described as a multiple-channel, two stage upconverter for processing television channel signals prior to broadcast in a cable television system. The first stage separates the frequencies of the channel signals so that those frequencies are spaced by the same frequency separation used between channels broadcast in the cable television system. The second stage then converts the channel signals to the frequencies for the desired channels in the VHF or UHF bands on which the signals will actually be broadcast. Consequently, the second-stage oscillator determines on which channels in the cable system the channel signals will be broadcast.
The upconverter of the present invention may be characterized by a number of inputs for receiving a corresponding number of channel signals at an intermediate frequency, a first converting stage and a second converting stage. Each of the two stages will be described in more detail below.
The first converting stage includes a number of mixers and oscillators with one mixer and oscillator being associated with and receiving each of the channel signals from the channel signal inputs. Each of the first-stage oscillators output an oscillator signal having a frequency different from the other oscillator signals output by the other first-stage oscillators. Preferably, the oscillator signals output by the first-stage oscillators have frequencies that are spaced apart by an amount equal to the frequency spacing between different channels in the cable television system. Each oscillator signal is mixed by an associated mixer with one of the channel signals. Preferably, the intermediate frequency of the channel signals, when received by the inputs for the first stage, is the same frequency for all of the channel signals.
In the second stage, the channel signals from the first stage are received and further processed. The second stage includes a number of second-stage mixers, each of which receives one of the channel signals from the first stage. Each of the second-stage mixers mixes a received channel signal with a second-stage oscillator signal output by a single second-stage oscillator that is connected in common to each of the second-stage mixers.
Band bass filtering and amplification of the channel signals may be performed before the channel signals are input to the first stage, between the first and second stages and after the channel signals are output by the second stage. Preferably, any such filtering and amplification stage includes a number of band pass filters each of which serves to filter one of the channel signals at a particular frequency to which that channel signal is provided or has been converted by the preceding converting stage.
After being processed by the second stage, the channel signals are combined by a combiner into a composite signal for broadcast over the cable television system. The upconverter of the present invention is particularly suited for video-on-demand services in which a number of channel signals can readily be upconverted and broadcast on a contiguous group of channels.
The present invention also encompasses the method of implementing the upconverter described above. For example, the present invention include the method of upconverting multiple television channel signals using a two stage technique prior to broadcast of the channel signals in a cable television system by, in a first stage, mixing each of the channel signals with one of a number of oscillator signals, where each of the oscillator signals has a frequency different than the other oscillator signals; and in a second stage, mixing each of the channel signals individually with a common second-stage oscillator signal.